This invention relates to electronic control circuits and particularly to circuits for controlling the drive of a voltage responsive device such as the input light emitting diode of an optical isolator.
Various means are known for responding to a control input signal to drive a voltage responsive element when the input is of a sufficient magnitude. For example, numerous control circuits use opto-isolators comprising, on the input side, a light emitting diode which responds to voltage by emitting radiation to which an output photodiode or phototransistor responds. In some applications, the energization of the light emitting diode desirably occurs only when the control input signal is above a defined threshold level. Among the arrangements used are those in which a voltage limiter such as a zener diode establishes the threshold level. Such arrangements are satisfactory for some purposes, but two features of their operation are disadvantages in certain applications. First, the use of a zener diode as the key voltage limiting element means that the input resistance is very high below the turn-on point, which makes the input sensitive to high impedance noise pulses. Additionally, at relatively high input voltage levels, the input current and, hence, power dissipation become excessive.
In copending application Ser. No. 608,227, filed June 21, 1976, by D. A. Fox and assigned to the assignee, there is disclosed a solid state relay which among its various features employs an opto-isolator unit for producing an isolated control signal for the input of a control logic circuit. The overall circuit of the copending application is one in which the present invention may be advantageously employed and it will be in the context of such a circuit that the present invention will be described although it will be understood that the circuit of the present invention may be applied elsewhere in the same or essentially similar form.